The present invention relates to a chromene compound which changes into a state of being colored in a yellowish to reddish color tone upon the irradiation with light containing ultraviolet rays such as of sunlight or light from a mercury lamp, the change being reversible, the color being quickly developed, the color density being high, and exhibiting little color when it is not developing color.
Photochromism is a phenomenon that is drawing attention in these several years, and is a reversible action of a compound which quickly changes its color when it is irradiated with light containing ultraviolet rays such as sunlight or light of a fluorescent lamp and resumes its initial color when it is no longer irradiated with light but is placed in a dark place. The compound having this property is called photochromic compound. Various compounds have so far been synthesized and have been used in a variety of fields.
When the photochromic compound is used as an optical material such as photochromic lenses for spectacles, a simple change in the color upon the irradiation with light or upon discontinuation of irradiation with light, is not enough. That is, a variety of properties are required, such as the material remains transparent (i.e., the photochromic compound has no initial color) in a state of not being irradiated with light, the material quickly develops color when it is irradiated with light in a desired color tone maintaining a sufficient degree of density, the color quickly fades away when the material is no longer irradiated with light, and the above properties are not deteriorated even after the material is used for extended periods of time (i.e., the material has a high durability).
In the field of photochromic spectacle lenses, in particular, a color tone that greatly affects the liking of the users serves as a very important factor, and it has been desired to maintain a neutral tint such as grey or brown which is considered to be the most desired color tone of lenses in this field of use not only when the color has been developed but also in a step of developing color and in a step in which the color fades away.
Many photochromic compounds have heretofore been proposed for photochromic lenses for spectacles, but no compound that satisfies the above requirements has been known yet.
As for the color tone, for example, it is difficult to obtain the above neutral tint relying upon one kind of photochromic compound only. Usually, the neutral tint is obtained by mixing a photochromic compound that develops yellowish to reddish color (hereinafter also referred to as yellow to red compound) and a photochromic compound that develops blue color (hereinafter also referred to as blue compound). Due to different color developing/fading rates between the two compounds, however, there occurs such a problem that the color tone becomes nonuniform at the time when the color is being developed or faded away.
That is, as the yellow to red compound, there has been known a chromene compound and as the blue compound, there has been known a spirooxazine-type compound or a fulgimide-type compound. In general, human eyes are more sensitive to blue than yellow or red. To obtain the above-mentioned uniform color tone, therefore, it is desired that the yellow to red compound develops color faster than the blue compound and, conversely, fades moderately slower than bule compound. The blue compound develops color at a relatively high rate, and there arises no problem when the color-developing rate of the yellow to red compound is comparable thereto. However, very few blue compounds that exhibit a high fading rate have been known. When the yellow to red compound having a high fading rate is used, therefore, the color tone becomes nonuniform at the time when the color fades away.
For example, PCT International Laid-Open Specification WO98/45281 discloses a chromene compound of the following formula (A) that develops orange color. 
This chromene compound, however, has both a low color developing rate and a low color fading rate. When used in combination with a known blue compound to obtain a composition that exhibits a neutral tint, therefore, this compound arouses a problem in that the color tone in the fading step becomes blue.
Japanese Unexamined Patent Publication (Kokai) No. 298176/1998 discloses a compound represented by the following formula (B), 
The color fading rate of this compound is lower than that of the above compound (A) but is not still satisfactory.
Further, U.S. Patent Specification No. 4980089 discloses a chromene compound represented by the following formula (C), 
Contrary to the above-mentioned two kinds of chromene compounds, however, this compound exhibits a too low color developing rate and a too low color fading rate, making it quite difficult to obtain uniformity in the step of developing color and in the step of fading color.
When it is attempted to obtain a neutral tint such as grey or brown by using the known chromene compound that develops yellow to red color in combination with the blue compound that has heretofore been used as described above, the color tone is not uniform in the step of developing color or in the step of fading color, and is not still satisfactory.
Besides, the photochromic compounds known thus far are not necessarily satisfactory with respect to the fading rate and the initial color.
For example, the chromene compound represented by the above general formula (A) disclosed in the above PCT International Laid-Open Specification WO98/45281 exhibits a relatively high fading rate but has a large initial color which casts a problem.
The chromene compound represented by the above general formula (B) disclosed in the above Japanese Unexamined Patent Publication (Kokai) No. 298176/1998 exhibits a relatively high fading rate and an improved initial color. The fading rate, however, is not still satisfactory when the chromene compound is used alone or when it is considered that the chromene compound will be used in combination with a blue compound having a very high fading rate that may be developed in the future.
It is therefore an object of the present invention to provide a novel chromene compound which exhibits at least one of the above-mentioned properties that are required when it is used for such applications as photochromic lenses for spectacles, the property being superior to that of the conventional photochromic compounds and, particularly, that of the conventional chromene compounds.
More concretely, the object of the present invention is to provide a chromene compound which has a high color density and a little initial color and which, when used in combination with a blue compound that is now being used, provides a uniform color tone in the steps of developing and fading color or, in other words, which exhibits nearly the same color developing rate as that of the blue compound that is now being used, exhibits a color fading rate which is low to a suitable degree, or exhibits a very high color fading rate.
In order to accomplish the above object, the present inventors have conducted keen study concerning the effect of when various substituents are introduced into the chromene compound. As a result, the present inventors have found the fact that the novel chromene compounds having a naphthopyran skeleton, particular substituents at the third and sixth positions of the naphthopyran ring and, as required, particular substituents at the seventh to tenth positions, exhibit a common feature in that a little initial color and a high color dencity exhibit a high color developing rate and a suitably low color fading rate depending upon the kind of the substituent, do not cause the color tone to lose balance at the step of fading when the compound is used being mixed together with the conventional blue compound, or exhibit a very high color fading rate, and have thus arrived at the present invention.
That is, the present invention is concerned with a chromene compound represented by the following general formula (1), 
wherein R1 is a substituted amino group, a substituted or unsubstituted heterocyclic group having a nitrogen atom, as a hetero atom, bonded to a naphthopyran ring through the nitrogen atom, or a condensed heterocyclic group in which said heterocyclic group is condensed with an aromatic hydrocarbon ring or an aromatic heterocyclic ring,
R2 is an alkyl group, an alkoxyl group, an aralkoxyl group, an aralkyl group, a substituted amino group, a cyano group, a substituted or unsubstituted aryl group, a halogen atom, a substituted or unsubstituted heterocyclic group having, as a hetero atom, a nitrogen atom, bonded to the naphthopyran ring through the nitrogen atom, or a condensed heterocyclic group in which said heterocyclic group is condensed with an aromatic hydrocarbon ring or an aromatic heterocyclic ring,
xe2x80x9caxe2x80x9d representing the number of the substituents R2 is an integer of 0 to 3,
X1 is a group represented by the following formula (2), 
wherein each of R3, R4 and R5 is a hydrogen atom, a substituted amino group, a substituted or unsubstituted heterocyclic groups, having a nitrogen atom, as a hetero atom, bonded to the benzene ring, or a condensed heterocyclic group in which said heterocyclic group is condensed with an aromatic hydrocarbon ring or an aromatic heterocyclic ring, but R3, R4 and R5 are not hydrogen atoms simultaneously, and
X2 is a group represented by the following formula (3), 
wherein R6 is a hydrogen atom; an electron attractive group selected from the group consisting of a trifluoromethyl group, a trifluoromethoxy group, a cyano group, a sulfonyl group, an alkylsulfonyl group, an arylsulfonyl group and a nitro group; or an alkoxyl group,
each of R7 and R8 is (i) a hydrogen atom, an aliphatic hydrocarbon group having not less than three carbon atoms, a halogen atom, a trifluoromethyl group, a trifluoromethoxy group, a cyano group, a sulfonyl group, an alkylsulfonyl group, an arylsulfonyl group or a nitro group when R6 is not a hydrogen atom, or (ii) a hydrogen atom, a halogen atom, a trifluoromethyl group or a trifluoromethoxy group when R6 is a hydrogen atom,
each of R9 and R10 is a hydrogen atom, a cyano group, an alkoxyl group having 1 to 5 carbon atoms, a fluorine atom or a chlorine atom,
wherein, when R4 in the group represented by the above formula (2) is the substituted amino group, the substituted or unsubstituted heterocyclic group having or condensed heterocyclic group, R6 is not an alkoxyl group, and R6, R7, R8, R9 and R10 are not hydrogen atoms simultaneously.
The above chromene compound has a feature of little initial color and a high density of color that is developed.
Among the chromene compounds of the present invention represented by the above general formula (1), the chromene compound in which:
R6, R7, R8, R9 and R10 in the above formula (3) are not hydrogen atoms simultaneously,
R6 is a hydrogen atom or an electron attractive group selected from the group consisting of a trifluoromethyl group, a cyano group, a sulfonyl group, an alkylsulfonyl group, an arylsulfonyl group and a nitro group,
when R6 is not a hydrogen atom, each of R7 and R8 is a hydrogen atom, an aliphatic hydrocarbon group having not less than 3 carbon atoms, a fluorine atom, a trifluoromethyl group, a cyano group, a sulfonyl group, an alkylsulfonyl group, an arylsulfonyl group or a nitro group, and
when R6 is a hydrogen atoms, each of R7 and R8 is a hydrogen atom,
exhibits a high color developing rate and a suitably low color fading rate in addition to the feature of little initial color and a high density of color that is developed, and does not cause the color tone to lose balance in the step of fading when it is used being mixed together with the conventional blue compound.
Among the chromene compounds of the present invention represented by the above general formula (1), further, the chromene compound in which:
R6 in the above formula (3) is a hydrogen atom, an alkoxyl group or a trifluoromethoxy group,
each of R7 and R8 is a hydrogen atom, a halogen atom, a trifluoromethyl group or a trifluoromethoxy group [wherein both R7 and R8 are not hydrogen atoms, when R4 in the formula (2) in the general formula (1) is the substituted amino group, the substituted or unsubstituted heterocyclic group or the condensed heterocyclic group], and
both R9 and R10 are hydrogen atoms, has a feature of a high fading rate in addition to the feature of little initial color and a high density of color that is developed.
Another invention is concerned with a photochromic material containing the chromene compound of the present invention, and a further invention is concerned with a photochromic optical material containing the chromene compound of the present invention.
According to the present invention, further, there is provided a photochromic polymerizable composition containing the above-mentioned chromene compound, a polymerizable monomer (particularly, a (meth)acrylic acid ester compound) and, as required, a polymerization initiator.